Positive electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

ABSTRACT

A positive electrode for a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector mainly composed of aluminum (Al), a protective layer disposed on the positive electrode current collector, and a positive electrode mixture layer containing a lithium-containing transition metal oxide and disposed on the protective layer. The protective layer has a thickness of 1 to 5 μm and contains an electroconductive material and an inorganic compound having an oxidation power lower than that of the lithium-containing transition metal oxide.

BACKGROUND

1. Technical Field

The present disclosure relates to a positive electrode for a nonaqueouselectrolyte secondary battery and a nonaqueous electrolyte secondarybattery.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2003-157852(Patent Literature 1) discloses a positive electrode for a lithiumbattery including an aluminum oxide coating film having a thickness of 1μm or less formed on the surface of an aluminum current collector.Patent Literature 1 describes that an aluminum oxide coating film havinga thickness exceeding 1 μm is not sufficiently broken when the currentcollector and the positive electrode mixture layer are pressed in thethickness direction, resulting in a significant deterioration in thecurrent collecting properties.

Incidentally, for example, an internal short circuit in a battery orexposure of a battery to high temperature may cause a redox reactionbetween a positive electrode active material and an aluminum currentcollector to cause large heat generation. Since the technology of PatentLiterature 1 cannot increase the thickness of the aluminum oxide coatingfilm, the heat generation due to such a redox reaction cannot besufficiently prevented.

SUMMARY

In one general aspect, the techniques disclosed here feature a positiveelectrode for a nonaqueous electrolyte secondary battery, the positiveelectrode comprising a positive electrode current collector mainlycomposed of aluminum (Al), a protective layer disposed on the positiveelectrode current collector, and a positive electrode mixture layercontaining a lithium-containing transition metal oxide and disposed onthe protective layer. The protective layer has a thickness of 1 to 5 μmand contains an electroconductive material and an inorganic compoundhaving an oxidation power lower than that of the lithium-containingtransition metal oxide.

The positive electrode for a nonaqueous electrolyte secondary batteryaccording to one aspect of the present disclosure can prevent heatgeneration due to a redox reaction between the positive electrode activematerial and the aluminum current collector, while maintainingsatisfactory current collecting properties.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondarybattery as an embodiment; and

FIG. 2 is a cross-sectional view of a positive electrode for anonaqueous electrolyte secondary battery as an embodiment.

DETAILED DESCRIPTION

The positive electrode for a nonaqueous electrolyte secondary battery(hereinafter, simply referred to as “positive electrode”) as anembodiment of the present disclosure includes a protective layerdisposed on a current collector, the protective layer having a thicknessof 1 to 5 μm and containing an electroconductive material and aninorganic compound having an oxidation power lower than that of alithium-containing transition metal oxide serving as a positiveelectrode active material. The present inventors have found that, forexample, an internal short circuit in a battery or exposure of a batteryto high temperature has a risk of causing a redox reaction between thepositive electrode active material (which is a lithium-containingtransition metal oxide) and the aluminum current collector (which ismainly composed of aluminum) to cause large heat generation. Theinventors have then developed a positive electrode including theabove-described protective layer in order to prevent the heat generationby such a redox reaction. The protective layer containing an inorganiccompound having an oxidation power lower than that of thelithium-containing transition metal oxide separates between the aluminumcurrent collector and the lithium-containing transition metal oxide toprevent the redox reaction in which the aluminum current collectorparticipates and thereby reduces the quantity of heat generated byoccurrence of abnormality.

In order to prevent the redox reaction, the protective layer needs tohave a thickness of at least 1 μm. The protective layer preferably has athickness of 1.5 μm or more and is preferably formed so as to have asurface density of 0.1 to 20 g/m². A simple increase in the thickness ofthe protective layer significantly decreases the current collectingproperties, as described in Patent Literature 1, to cause adeterioration in battery performance. The present inventors, however,have found that an addition of an electroconductive material to theprotective layer can secure the current collecting properties. That is,the positive electrode as an embodiment of the present disclosure canprevent heat generation due to a redox reaction between the positiveelectrode active material and the aluminum current collector, whilemaintaining satisfactory current collecting properties.

An embodiment of the present disclosure will now be described in detail.

The drawings referred to in explanation of the embodiment are schematic,and the dimension ratios of components and other factors shown in thedrawings may be different from those of actual one. Specific dimensionalratios and other factors should be judged from the followingdescriptions.

FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondarybattery 10 as an embodiment.

The nonaqueous electrolyte secondary battery 10 includes a positiveelectrode 11, a negative electrode 12, and a nonaqueous electrolyte. Aseparator 13 is preferably provided between the positive electrode 11and the negative electrode 12. The nonaqueous electrolyte secondarybattery 10 has a structure, for example, in which a wound-type electrodeassembly 14 produced by winding the positive electrode 11 and thenegative electrode 12 with the separator 13 therebetween and thenonaqueous electrolyte are accommodated in a battery container. Insteadof the wound-type electrode assembly 14, other electrode assemblies,such as a lamination-type electrode assembly composed of positiveelectrodes and negative electrodes alternately laminated with separatorstherebetween, may be employed. Examples of the battery containeraccommodating the electrode assembly 14 and the nonaqueous electrolyteinclude cylindrical, rectangular, coin, and button-shaped metalcontainers and resin containers (laminate-type batteries) formed bylaminating resin sheets. In the example shown in FIG. 1, the batterycontainer is composed of a bottomed cylindrical container body 15 and asealing body 16.

The nonaqueous electrolyte secondary battery 10 includes insulatingplates 17, 18 respectively disposed at the top and the bottom of theelectrode assembly 14. In the example shown in FIG. 1, a positiveelectrode lead 19 is attached to the positive electrode 11, and anegative electrode lead 20 is attached to the negative electrode 12. Thepositive electrode lead 19 passes through a through-hole of theinsulating plate 17 and extends to the sealing body 16 side, and thenegative electrode lead 20 extends to the bottom side of the containerbody 15 through the outside of the insulating plate 18. For example, thepositive electrode lead 19 is connected, by, for example, welding, tothe lower surface of a filter 22 serving as the basal plate of thesealing body 16, and thereby a cap 26 serving as the top plate of thesealing 16 to which the filter 22 is electrically connected functions asa positive electrode terminal. The negative electrode lead 20 connected,by, for example, welding, to the inner surface of the bottom of thecontainer body 15, and thereby the container body 15 functions as anegative electrode terminal. In the embodiment, the sealing body 16 isprovided with a current interruption device (CID) and a gas dischargemechanism (safety valve). The bottom of the container body 15 is alsopreferably provided with a gas discharge valve (not shown).

The container body 15 is, for example, a bottomed cylindrical metalcontainer. A gasket 27 is disposed between the container body 15 and thesealing body 16 to secure the sealing performance of the batterycontainer. The container body 15 preferably has a protrusion part 21that is formed by, for example, pressing the side wall from the outsideand supports the sealing body 16. The protrusion part 21 is preferablyformed in a ring shape along the circumferential direction of thecontainer body 15 and supports the sealing body 16 with the uppersurface thereof.

The sealing body 16 includes a filter 22 having a filter opening 22 aand a valve element disposed on the filter 22. The valve elementoccludes the filter opening 22 a of the filter 22 and is broken when theinner pressure of the battery is increased by heat generation due to aninternal short circuit or another reason. In the embodiment, the sealingbody includes a lower valve element 23 and an upper valve element 25 asthe valve elements, and also includes an insulating member 24 disposedbetween the lower valve element 23 and the upper valve element 25 and acap 26 having a cap opening 26 a. Each member constituting the sealingbody 16 has, for example, a disk or ring-like shape, and the membersexcluding the insulating member 24 are electrically connected to oneanother. Specifically, the filter 22 and the lower valve element 23 arebonded to each other at the periphery thereof, and the upper valveelement 25 and the cap 26 are also bonded to each other at the peripherythereof. The lower valve element 23 and the upper valve element 25 areconnected to each other at the central portion thereof, and theinsulating member 24 is disposed between the lower valve element 23 andthe upper valve element 25 at the periphery thereof. If the innerpressure is increased by heat generation due to an internal shortcircuit or another reason, for example, the lower valve element 23 has asmall thickness and is broken. As a result, the upper valve element 25expands towards the cap 26 side and separates from the lower valveelement 23 to break the electrical connection between them.

[Positive Electrode]

FIG. 2 is a cross-sectional view of a positive electrode 11 as anembodiment.

The positive electrode 11 includes a positive electrode currentcollector 30 mainly composed of aluminum (A), a protective layer 31disposed on the positive electrode current collector 30, and a positiveelectrode mixture layer 32 containing a lithium-containing transitionmetal oxide and disposed on the protective layer 31. Here, the term“being mainly composed” refers to that the proportion of the material isthe highest among the materials. The positive electrode mixture layer 32contains the lithium-containing transition metal oxide as a positiveelectrode active material and preferably further contains anelectroconductive material and a binding material. The positiveelectrode 11 can be produced by, for example, applying a positiveelectrode mixture slurry containing a positive electrode activematerial, a binding material, and other materials onto each protectivelayer 31 formed on a positive electrode current collector 30; drying thecoating films; and then performing rolling. Thus, a positive electrodemixture layer 32 is formed on both surfaces of the current collector.

In the positive electrode current collector 30, for example, aluminum oran aluminum alloy is used. The content of aluminum in the positiveelectrode current collector 30 is 50% or more, preferably 70% or more,and more preferably 80% or more, based on the total weight of thecurrent collector. The positive electrode current collector 30 is, forexample, metal foil made of aluminum or an aluminum alloy and having athickness of about 10 to 100 μm.

The positive electrode active material is, for example, alithium-transition metal oxide containing a transition metal elementsuch as Co, Mn, or Ni. Examples of the lithium-transition metal oxideinclude Li_(x)CoO₂, Li_(x)NiO₂, Li_(x)MnO₂, Li_(x)Co_(y)Ni_(1-y)O₂,Li_(x)Co_(y)M_(1-y)O_(z), Li_(x)Ni_(1-y)M_(y)O_(z), Li_(x)Mn₂O₄,Li_(x)Mn_(2-y)M_(y)O₄, LiMPO₄, and Li₂MPO₄F (M; at least one of Na, Mg,Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0<x≦1.2, 0<y≦0.9,and 2.0≦z≦2.3). These lithium-transition metal oxides may be used aloneor as a mixture of two or more thereof.

The electroconductive material contained in the positive electrodemixture layer 32 enhances the electrical conductivity of the positiveelectrode mixture layer. Examples of the electroconductive materialinclude carbon materials such as carbon black (CB), acetylene black(AB), Ketjen black, and graphite. These electroconductive materials maybe used alone or in combination of two or more thereof.

The binding material contained in the positive electrode mixture layer32 functions so as to maintain the good contact between the positiveelectrode active material and the electroconductive material and toenhance the binding property of, for example, the positive electrodeactive material to the surface of the positive electrode currentcollector. Examples of the binding material include fluororesins, suchas polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), polyimide resins, acrylic resins, andpolyolefin resins. These resins may be used in combination with, forexample, carboxymethyl cellulose (CMC) or its salt (which is, forexample, CMC-Na, CMC-K, or CMC-NH₄ or may be a partially neutralizedsalt) or polyethylene oxide (PEO). These binding materials may be usedalone or in combination of two or more thereof.

The positive electrode 11 includes a protective layer 31 between thepositive electrode current collector 30 and the positive electrodemixture layer 32, as described above. The protective layer 31 separatesbetween the positive electrode current collector 30 mainly made ofaluminum and the lithium-transition metal oxide serving as the positiveelectrode active material and prevents the redox reaction in which thepositive electrode current collector 30 participates.

The protective layer 31 has a thickness of 1 to 5 μm and contains aninorganic compound (hereinafter, referred to as “inorganic compound P”)having an oxidation power lower than that of the lithium-containingtransition metal oxide contained in the positive electrode mixture layer32 and also contains an electroconductive material. Since the protectivelayer 31 contains the electroconductive material in addition to theinorganic compound P, the positive electrode 11 can secure good currentcollecting properties. The inorganic compound P is, for example,particles having an average particle diameter (volume-average particlediameter measured by a light scattering method) of 1 μm or less. Theprotective layer 31 preferably contains a binding material for securingits mechanical strength by binding the inorganic compound P and theelectroconductive material and for enhancing the binding propertybetween the protective layer 31 and the positive electrode currentcollector 30.

Preferred examples of the inorganic compound P include inorganic oxides,such as manganese oxide, silicon dioxide, titanium dioxide, and aluminumoxide. In particular, aluminum oxide is preferred. The content of theinorganic compound P is preferably 70% to 99.8% by weight andparticularly preferably 90% to 99% by weight based on the total weightof the protective layer 31. A content of the inorganic compound P withinthe above-mentioned range enhances the effect of preventing the redoxreaction to easily reduce the quantity of heat generated by occurrenceof abnormality.

The protective layer 31 is formed on the positive electrode currentcollector 30, preferably so as to have a surface density of 0.1 to 20g/m². A protective layer 31 having a surface density within this rangecan sufficiently prevent contact between the positive electrode currentcollector 30 and the lithium-transition metal oxide to easily reduce thequantity of heat generated by occurrence of abnormality. The protectivelayer 31 is particularly preferably formed so as to have a thickness of1.5 to 5 μm and a surface density of 1 to 10 g/m². The protective layer31 can be formed by, for example, applying a slurry containing aninorganic compound P, an electroconductive material, and a bindingmaterial onto a positive electrode current collector 30; and drying thecoating film. When the positive electrode mixture layer 32 is providedon both surfaces of the positive electrode current collector 30, theprotective layer 31 is also provided on both surfaces of the positiveelectrode current collector 30.

The electroconductive material contained in the protective layer 31 canbe the same type as that of the electroconductive material applied tothe positive electrode mixture layer 32, for example, can be a carbonmaterial such as carbon black (CB), acetylene black (AB), Ketjen black,or graphite. These electroconductive materials may be used alone or incombination of two or more thereof. The content of the electroconductivematerial is preferably 0.1% to 20% by weight and particularly preferably1% to 10% by weight, based on the total weight of the protective layer31. The content rate of the electroconductive material in the protectivelayer 31 is higher than that of the electroconductive material in, forexample, the positive electrode mixture layer 32.

The binding material contained in the protective layer 31 can be thesame type as that of the electroconductive material applied to thepositive electrode mixture layer 32, for example, can be a fluororesinsuch as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride(PVdF), polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, ora polyolefin resin. These binding materials may be used alone or incombination of two or more thereof. The content of the binding materialis preferably 0.1% to 20% by weight and particularly preferably 1% to10% by weight, based on the total weight of the protective layer 31.

[Negative Electrode]

The negative electrode is composed of a negative electrode currentcollector of, for example, metal foil and a negative electrode mixturelayer disposed on the current collector. The negative electrode currentcollector can be, for example, metal foil that is stable in thepotential range of the negative electrode, such as copper, or a filmhaving a surface layer of such a metal. The negative electrode mixturelayer preferably contains a binding material, in addition to a negativeelectrode active material. The negative electrode can be produced by,for example, applying a negative electrode mixture slurry containing anegative electrode active material, a binding material, and othermaterials onto a negative electrode current collector; drying thecoating film; and then performing rolling. Thus, a negative electrodemixture layer is formed on both surfaces of the current collector.

The negative electrode active material may be any material that canreversibly occlude and discharge lithium and can be, for example, acarbon material, such as natural graphite or artificial graphite; amaterial alloying with lithium, such as silicon (Si) or tin (Sn); or analloy or complex oxide containing a metal element such as Si or Sn. Thenegative electrode active materials may be used alone or in combinationof two or more thereof.

The binding material contained in the negative electrode mixture layercan be, as in the negative electrode, a fluororesin, PAN, a polyimideresin, an acrylic resin, or a polyolefin resin. In the case of using anaqueous solvent for preparing the negative electrode mixture slurry, forexample, styrene-butadiene rubber (SBR), CMC or its salt, polyacrylicacid (PAA) or its salt (which is, for example, PAA-Na or PAA-K or may bea partially neutralized salt), or polyvinyl alcohol (PVA) is preferablyused.

[Separator]

The separator used is a porous sheet having ionic permeability andinsulation properties. Examples of the porous sheet include micro-porousthin films, woven fabric, and non-woven fabric. Preferred materials ofthe separator are, for example, olefin resins, such as polyethylene andpolypropylene, and cellulose. The separator may be a laminate includinga cellulose fiber layer and a thermoplastic resin fiber layer made of,for example, a olefin resin. The separator may be a multilayer separatorincluding a polyethylene layer and a polypropylene layer or may have asurface onto which an aramid resin is applied.

A filler layer containing an inorganic filler may be disposed betweenthe separator and the positive electrode and/or between the separatorand the negative electrode. Examples of the inorganic filler includeoxides containing at least one selected from titanium (Ti), aluminum(Al), silicon (Si), and magnesium (Mg); and phosphate compounds. Thefiller layer can be formed by, for example, applying a slurry containingthe filler onto the surface of the positive electrode, negativeelectrode, or separator.

[Nonaqueous Electrolyte]

The nonaqueous electrolyte includes a nonaqueous solvent and anelectrolyte salt dissolved in the nonaqueous solvent. The nonaqueouselectrolyte is not limited to liquid electrolytes (nonaqueouselectrolytic solutions) and may be a solid electrolyte, such as gelledpolymers. Examples of the nonaqueous solvent include esters, ethers,nitriles such as acetonitrile, amides such as dimethylformamide, andsolvent mixtures of two or more thereof. The nonaqueous solvent maycontain a halogen-substituted derivative having at least a part ofhydrogen atoms of the solvent substituted with halogen atoms such asfluorine atoms.

Examples of the esters include cyclic carbonate esters, such as ethylenecarbonate (EC), propylene carbonate (PC), and butylene carbonate; chaincarbonate esters, such as dimethyl carbonate (DMC), ethyl methylcarbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethylpropyl carbonate, and methyl isopropyl carbonate; cyclic carboxylateesters, such as γ-butyrolactone and γ-valerolactone; and chaincarboxylate esters, such as methyl acetate, ethyl acetate, propylacetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

Examples of the ethers include cyclic ethers, such as 1,3-dioxolane,4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran,propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane,1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ether; andchain ethers, such as 1,2-dimethoxyethane, diethyl ether, dipropylether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinylether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butylphenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether,diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane,1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycoldiethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane,1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethyleneglycol dimethyl ether.

Preferred examples of the halogen-substituted derivative includefluorinated cyclic carbonate esters, such as fluoroethylene carbonate(FEC); fluorinated chain carbonate esters; and fluorinated chaincarboxylate esters, such as methyl fluoropropionate (FMP).

The electrolyte salt is preferably a lithium salt. Examples of thelithium salt include LiBF₄, LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiAlCl₄,LiSCN, LiCF₃SO₃, LiCF₃CO₂, Li(P(C₂O₄)F₄), LiPF_(6-x)(C_(n)F_(2n+1))_(x)(1<x<6, n=1 or 2), LiB₁₀Cl₁₀, LiCl, LiBr, Lil, chloroborane lithium,lower aliphatic lithium carboxylate, borates such as Li₂B₄O₇ andLi(B(C₂O₄)F₂), and imides such as LiN(SO₂CF₃)₂ andLiN(C₁F_(2l+1)SO₂)(C_(m)F_(2m+1)SO₂) (where, l and m each represent aninteger of 1 or more). Lithium salts may be used alone or as a mixtureof two or more thereof. Among these lithium salts, from the viewpoint ofionic conductivity, electrochemical stability, and other factors, LiPF₆is preferably used. The concentration of the lithium salt is preferably0.8 to 1.8 mol per 1 L of the nonaqueous solvent.

EXAMPLES

The present disclosure will now be more specifically described byExamples, but is not limited to the following Examples,

Example 1 Production of Positive Electrode

Aluminum oxide (Al₂O₃, 93.5 parts by weight), acetylene black (AB, 5parts by weight), and polyvinylidene fluoride (PVdF, 1.5 parts byweight) were mixed, and the mixture was further mixed with anappropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a slurry.The slurry was then applied onto both surfaces of a positive electrodecurrent collector of aluminum foil having a thickness of 15 μm, anddrying is performed to form a protective layer having a thickness of 3.0μm and a surface density of 5.0 g/m².

Lithium-containing transition metal oxide (97 parts by weight)represented by LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ and serving as a positiveelectrode active material, acetylene black (AB, 2 parts by weight), andpolyvinylidene fluoride (PVdF, 1 part by weight) were mixed, and anappropriate amount of N-methyl-2-pyrrolidone (NMP) was added to themixture to prepare a positive electrode mixture slurry. The positiveelectrode mixture slurry was then applied to the surfaces of theprotective layers formed on the positive electrode current collector,followed by drying. The resulting product was cut into a prescribedelectrode size and was rolled with a roller to produce a positiveelectrode composed of the positive electrode current collector providedwith a protective layer and a positive electrode mixture layer in thisorder on both surfaces.

[Production of Negative Electrode]

A graphite powder (98.7 parts by weight), carboxymethyl cellulose (CMC,0.7 parts by weight), and styrene-butadiene rubber (SBR, 0.6 parts byweight) were mixed, and an appropriate amount of water was further addedto the mixture to prepare a negative electrode mixture slurry. Thenegative electrode mixture slurry was then applied onto both surfaces ofa negative electrode current collector of copper foil, followed bydrying. The resulting product was cut into a prescribed electrode sizeand was rolled with a roller to produce a negative electrode composed ofthe negative electrode current collector provided with a negativeelectrode mixture layer on both surfaces.

[Production of Nonaqueous Electrolyte]

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethylcarbonate (DMC) were mixed at a volume ratio of 3:3:4. LiPF₆ wasdissolved in this solvent mixture at a concentration of 1.2 mol/L toprepare a nonaqueous electrolytic solution.

[Production of Battery]

An aluminum lead and a nickel lead were attached to the positiveelectrode and the negative electrode, respectively. The positiveelectrode and the negative electrode were spirally wound with apolyethylene separator therebetween to produce a wound electrodeassembly. The electrode assembly was accommodated in a bottomedcylindrical battery container body having an outer diameter of 18.2 mmand a height of 65 mm, and the nonaqueous electrolytic solution preparedabove was poured therein. The opening of the battery container body wasthen sealed by the gasket and a sealing body to produce 18650-typecylindrical nonaqueous electrolyte secondary battery A1.

Example 2

Battery A2 was produced as in Example 1 except that the protective layerhad a thickness of 1.5 μm and a surface density of 2.5 g/m².

Example 3

Battery A3 was produced as in Example 1 except that the protective layerhad a thickness of 1.0 μm and a surface density of 1.6 g/m².

Example 4

Battery A4 was produced as in Example 1 except that the weight ratio ofthe constituent materials of the protective layer was aluminumoxide/electroconductive material/binding material=50/45/5 and that theprotective layer had a thickness of 5.0 μm and a surface density of 5.0g/m².

Comparative Example 1

Battery B1 was produced as in Example 1 except that the protective layerwas not formed.

Comparative Example 2

Battery B2 was produced as in Example 1 except that theelectroconductive material was not added to the protective layer andthat the amount of the aluminum oxide was increased instead of theelectroconductive material.

Comparative Example 3

Battery B3 was produced as in Example 1 except that the protective layerhad a thickness of 0.5 μm and a surface density of 0.7 g/m².

[Measurement of Battery Capacity]

The battery capacity of each battery produced above was measured by thefollowing procedure:

Each battery was charged with a constant current of 0.3 It (600 mA) to abattery voltage of 4.2 V at an environmental temperature of 25° C. andwas then charged at a constant voltage of 4.2 V. Subsequently, eachbattery was discharged at a constant current of 0.3 It (600 mA) untilthe battery voltage reached 3.0 V. The discharging capacity on thisoccasion was determined as the battery capacity.

[Measurement of Internal Resistance]

The internal resistance of each battery produced above was measured bythe following procedure:

Each battery was charged with a constant current of 0.3 It (600 mA) to abattery voltage of 4.2 V at an environmental temperature of 25° C. andwas then charged at a constant voltage of 4.2 V. Subsequently, theinterterminal resistance of each battery was measured with alow-resistance meter (at a measurement frequency of 1 kHz by an ACfour-probe method). The resistance value on this occasion was determinedas the internal resistance.

[Nail Sticking Test]

Each battery was tested by the following procedure:

(1) Each battery was charged with a constant current of 0.3 C (600 mA)to a battery voltage of 4.2 V at an environmental temperature of 25° C.and was then continuously charged at a constant voltage of 4.2 V until acurrent value of 0.05 C (90 mA).

(2) The tip of a wire nail having a diameter of 3 mm was brought intocontact with the central portion of the side face of the battery chargedin the step (1) under an environment of a temperature of 25° C. The wirenail was stuck into the battery at a rate of 10 mm/sec along thediameter direction, and the sticking was stopped when the wire nailcompletely pierced the battery.

(3) The battery temperature was measured at a position 10 mm apart fromthe central portion of the side face of the battery at which the wirenail was stuck to determine the highest temperature of the battery.

TABLE 1 Battery Nail sticking test Protective layer characteristicsHighest Surface Battery internal temperature density Thickness Weightratio capacity resistance of battery Battery (g/m²) (μm) Al₂O₃/AB/PVdF(mAh) (mΩ) (° C.) A1 5.0 3.0 93.5/5/1.5 1556 15.7 586 A2 2.5 1.593.5/5/1.5 1568 15.5 622 A3 1.6 1.0 93.5/5/1.5 1570 15.5 654 A4 5.0 5.050/45/5 1545 15.4 711 B1 — — — 1582 15.2 744 B2 5.0 3.0 98.5/0/1.5 112254.7 551 B3 0.7 0.5 93.5/5/1.5 1580 15.3 741

The results shown in Table 1 demonstrate that in the batteries ofExamples each including a protective layer containing aluminum oxide andan electroconductive material and having a thickness of 1 to 5 μm and asurface density of 1.6 to 5.0 g/m² on the aluminum current collector ofthe positive electrode, the heat generation by occurrence of abnormalitysuch as nail sticking is drastically prevented. This result is probablycaused by that the protective layer prevents the redox reaction betweenthe positive electrode active material (which is the lithium-containingtransition metal oxide) and the aluminum current collector.

In addition, the batteries of Examples have battery capacities andinternal resistances similar to those of the battery (ComparativeExample 1) not including any protective layer. That is, it is understoodthat the battery characteristics, such as battery capacity and internalresistance, are not deteriorated even if the protective layers aredisposed as in Examples. However, in a protective layer not containingany electroconductive material (Comparative Example 2) or having a smallthickness of 0.5 urn (Comparative Example 3) cannot simultaneouslyachieve satisfactory battery characteristics and heatgeneration-preventing effect. In the former case, a reduction in thebattery capacity and an increase in the internal resistance occur. Inthe latter case, the effect of preventing heat generation due tooccurrence of abnormality is low. That is, satisfactory batterycharacteristics and high heat generation-preventing effect can besimultaneously achieved only when a protective layer is employed as inExamples.

What is claimed is:
 1. A positive electrode for a nonaqueous electrolyte secondary battery, the positive electrode comprising; a positive electrode current collector mainly composed of aluminum (Al); a protective layer disposed on the positive electrode current collector; and a positive electrode mixture layer containing a lithium-containing transition metal oxide and disposed on the protective layer, wherein the protective layer has a thickness of 1 to 5 μm and contains an electroconductive material and an inorganic compound having an oxidation power lower than that of the lithium-containing transition metal oxide.
 2. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the inorganic compound is aluminum oxide.
 3. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer is formed on the positive electrode current collector at a surface density of 0.1 to 20 g/m².
 4. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer contains the inorganic compound in an amount of 70% to 99.8% by weight based on the total weight of the protective layer.
 5. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer contains the electroconductive material in an amount of 0.1% to 20% by weight based on the total weight of the protective layer.
 6. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer further contains a binding material in an amount of 0.1% to 10% by weight based on the total weight of the protective layer.
 7. A nonaqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; and a nonaqueous electrolyte, wherein the positive electrode comprises: a positive electrode current collector mainly composed of aluminum (Al); a protective layer disposed on the positive electrode current collector; and a positive electrode mixture layer containing a lithium-containing transition metal oxide and disposed on the protective layer, and the protective layer has a thickness of 1 to 5 and contains an electroconductive material and an inorganic compound having an oxidation power lower than that of the lithium-containing transition metal oxide. 